Re: Problem with "NA"

[Terry Duell]

Hello Juan,

On Tue, 19 Feb 2013 18:25:47 +1100, Juan Pablo Carbajal  
<address@hidden> wrote:

> On Tue, Feb 19, 2013 at 7:05 AM, Terry Duell <address@hidden> wrote:
> > Hello All,
> > I am solving a number of simultaneous DEs.
> > If I run the solution for a time t1 (say) I get quite respectable  
> > results
> > for all my derivatives up to a time that is a bit short of t1. The  
> > results
> > for the rest of the time are "NA". Thus far I can't see why this is
> > happening.
> > What sort of circumstances usually cause NAs ?
> >
> > Cheers,
> > --
> > Regards,
> > Terry Duell
> > _______________________________________________
> > Help-octave mailing list
> > address@hidden
> > https://mailman.cae.wisc.edu/listinfo/help-octave
>
> NA is for missing data
> http://www.gnu.org/software/octave/doc/interpreter/Missing-Data.html
> bad oprations like division by zero and such should produce NaN.


OK, that's interesting. I'm not getting any division by zero warnings.

> Could you give us more details about your particular setup? What
> solver are you using?

It is a 7 DOF system, and I'm using a bespoke RK solver with fixed time  
steps.

> Are you solving a nonlinear equation?

I'm not sure of the strict definition of non linear in this context.
The system is I'm investigating is a 2 axle vehicle with a single axle  
trailer attached by a hinge joint.
All three axles have variable rate tyre springs, and variable rate  
suspension springs and dampers.

> Does the solution exist for your initial conditions and your time window?

I think so, or I think it should. The simulation looks at the system  
response while being excited by a road surface with constant statistics,  
hence if the system is numerically stable, it should produce pretty much  
the same response for a long time period as it does for a short one.
If I run the system as a solo vehicle (ie no trailer) it behaves as  
expected and produces results comparable with those from other  
simulations. If I run the vehicle combination, all the results I have  
looked at thus far look good with no signs of any numerical instability,  
right up to a short time prior to the end of the simulation period.  
Thereafter the results are all NA.

If division by zero is the most likely cause, are there situations where  
division by zero warnings can be missed or hidden? To put it another way,  
is there a way of ensuring division by zero warnings are captured and  
reported?

Cheers,
--
Regards,
Terry Duell